1. Field of the Invention
The invention relates to the treatment of sour petroleum distillates. Specifically, the invention relates to a catalyst for sweetening sour petroleum distillates by using an oxometallo chelate catalyst to oxidize mercaptans in the distillates to disulfides. The catalyst may also be used to oxidize inorganic sulfides to elemental sulfur.
2. Description of the Prior Art
Several processes are currently used in the petroleum refining industry to sweeten sour petroleum distillates. Sweetening refers to the oxidation of offensive mercaptans contained in petroleum distillates to disulfides. The objectionable properties of mercaptans include their foul odors, toxicities, and corrosive behavior to metals. Sour petroleum distillates include straight-run gasoline, cracked gasoline, kerosene, jet fuel, naptha, fuel oil, gaseous fractions, and the like.
One type of sweetening process utilizes an oxidizing agent, usually air, and an oxidation catalyst, usually a metal phthalocyanine to sweeten the sour distillates. A general feature of such process is the requirement of a strongly basic medium to effect the oxidation reaction as disclosed in U.S. Pat. No. 2,882,224. In such process, a metal phthalocyanine chelate catalyst is contained in an alkaline aqueous solution, which is contacted with a sour petroleum distillate and air. The oxidation catalyst can be retained in the alkaline aqueous phase by incorporating suitable substituent groups on the phthalocyanine chelate. Thus, suitable catalysts include mono- and polysulfonated metallo phthalocyanines.
Another sweetening process is disclosed in U.S. Pat. No. 2,988,500, wherein a metallo phthalocyanine catalyst is composited on a solid support, such as charcoal, and an oxidizing agent, aqueous caustic solution, and sour petroleum distillate are passed over the composited catalyst.
Another sweetening process is described in U.S. Pat. No. 4,207,173, wherein an organic base, such as tetra-alkyl guanidine, is employed as the basic medium. The guanidine is added to the sour petroleum feed stream, which admixed with air is passed over a metallo phthalocyanine catalyst composited on a carbon carrier.
Another manner by which the sweetening of sour petroleum distillates can be achieved is disclosed by U.S. Pat. No. 4,260,479, wherein the basic medium is provided by a quaternary ammonium hydroxide, which is preferably composited with a metallo phthalocyanine catalyst on a carbon carrier.
The chemistry of hydrogen sulfide, which may be found in the lower boiling or gaseous petroleum fractions, or of alkali-metal salts of sulfides in aqueous solutions, such as sodium sulfide in waste water, is sufficiently similar to mercaptan oxidation chemistry so that petroleum sweetening catalysts have been directly and easily applied to processes that oxidize these inorganic sulfides to elemental sulfur. Hydrogen sulfide and alkali-metal sulfide salts are objectionable for similar reasons as mercaptans, namely their toxicity, foul odor, corrosive tendency, and gum or sludge forming or causing tendency.
In each of the petroleum sweetening processes of the prior art, the catalyst used consists of a metal-chelate compound. The preferred metal in the prior art is cobalt, while the preferred chelate is phthalocyanine or a structurally similar chelate such as porphyrin or tetrapyridinoporphyrazine. The use of porphyrins is described in U.S. Pat. No. 2,966,453 and the use of tetrapyridinoporphyrazines is described in U.S. Pat. No. 3,980,582. These chelates are similar in that all of the atoms of the chelate that are in close proximity to the metal atom are coplanar with the metal atom and each other. Thus, the metal atom is bonded by four nitrogen atoms in a square planar coordination environment. This environment results in catalytic activity of the metal atom. An examination of this square planar structure reveals that there are two additional coordination sites available to the metal atom, neither of which is in the metal-chelate plane. Instead these positions are above and below the metal chelate plane. Ligands at these positions form an axis, together with the metal atom, through and perpendicular to the metal-chelate plane. Such ligands will be referred to as axial ligands. ##STR1## In the prior art the usual catalyst has been a cobalt phthalocyanine compound. This metal chelate generally does not bond to axial ligands, and no use of such axial ligands in petroleum sweetening catalysts has been considered in the prior art.
There are a number of variations of the sweetening process of sour petroleum distillates using a metal phthalocyanine or similar catalysts and a basic environment. The use of a basic environment, however, has been the case of numerous problems such as disclosed in U.S. Pat. No. 4,207,173. These problems include the formation of soaps which plug the charcoal catalyst bed; the contamination of the final distillate product with either sodium hydroxide or water, or both; formation of emulsions from sodium salt that carry water into the final product; and the cost of replacing and disposing of the caustic solution which is required when the solution eventually becomes contaminated with toxins or catalyst poisons extracted from the distillate.
Other methods that employ organic bases to supply the basic medium have their own problems. Organic bases tend to be more expensive than aqueous caustic solutions due to the greater cost of such organics over aqueous caustic. If organic bases such as tetra-alkyl guanidines are used without an additional aqueous phase, they must be added in proportion to the amount of mercaptan contained in the sour distillate, increasing the cost of their use. When composited on a fixed bed with a metal chelate catalyst, the amount of base becomes depleted as the treatment of sour distillate proceeds. Alternatively, the organic base may be incorporated in aqueous solutions, in which case they are used as the caustic solution in practicing such methods. Thus, while the prior art has examined many ways to accommodate the problems caused by using organic or inorganic bases, no method is known that does not require the use of base in addition to the metal chelate catalyst.